Flexible blade switches have two metal blades whose ends come into mutual contact which closes a circuit in which the switch is mounted when they are exposed to an appropriate magnetic field. Because of the elasticity of the blades, the switch opens again when the magnetic field disappears.
Such a switch has been mounted in the "dry" part of a fluid meter, i.e., the part located outside of its hydraulic part, and if it is placed opposite to the trajectory of a magnet fixedly attached to the meter shaft but offset with respect thereto, the magnet passes by the switch one time for each revolution of the shaft and closes it for a brief instant.
It is sufficient to count the number of times the switch closes to deduce the number of revolutions the shaft makes. The volume of fluid which has passed through the meter is easily determined by using a meter which rotates proportionally to the volume of the fluid passing through it. The number of switch closing may be recorded by electronic registers well-known to those skilled in the art and can perform such operations, and therefore will not be described here.
The gauges described above were next improved by adding a second flexible blade switch, also located on the circular path of the magnetic field, but angularly offset 90.degree. from the first, which also makes it possible to detect the direction in which the shaft is rotating. Clearly, depending on the rotational direction, the second switch will close after the first at a 90.degree. or 270.degree. angular interval. As a result, there is a time interval between these two closing instants which is greater than or less than the following one. One of ordinary skill in the art can easily buy or design a logic circuit to compare this time interval and use this comparison to determine the rotational direction.
These known gauges nonetheless present a number of disadvantages, the most significant of which is related to flexible blade switch itself. Because the blades of said switches tend to undergo a rebound effect owing to their elasticity, the measurements may be inaccurate especially when the meter shaft is rotating at a high speed. Other perturbations can result from the dimensional inaccuracy and drift over time which occurs in flexible blade switches. Therefore, one may have an inaccurate measurement of the number or direction of rotations.